Physiological Entomology (2010) 35, 222–230 DOI: 10.1111/j.1365-3032.2010.00739.x Size compensation in moth larvae: attention to larval instars T O O M A S E S P E R K and T O O M A S T A M M A R U Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia Abstract. Environmental perturbations such as starvation and poor diet often prevent animals from attaining their optimal sizes. When the perturbation has a transient char- acter, compensatory responses are expected in terms of faster growth or a prolonged developmental period. In the case of insect larvae, details of such responses are insuf- ficiently known at the proximate level. Attention to responses at the level of particular larval instars should promote an understanding of insect developmental plasticity also in a more general context. To provide an instar-specific analysis of compensatory growth, larvae of the moth Orgyia antiqua (L.) are reared on inferior diet during one larval instar. Responses in growth parameters are recorded in the course of the manipulated instars, as well as at the level of the entire larval period. The negative relationship between development time and size in response to the inferior food quality, typical of the entire larval periods, is also observed within the manipulated instars taken separately. The manipulated larvae remain smaller than the larvae of the control group (significant in males only), even by the end of the subsequent instar during which all individuals are provided with superior host. In males, close to full size compensation by the time of pupation is achieved only by means of adding an extra larval instar. The inability of larvae to fully compensate during one and even two instars is considered as an indication of the presence of constraints on the within- instar growth pattern. An alternative, adaptational explanation for the incomplete compensation could be based on the cost of prolonged development period. Given the ecological context of the species’ life history, such an explanation appears less likely. Key words. Compensatory responses, growth curve, Lepidoptera, life history, moult, Orgyia. Introduction Poor quality of nutritional resources leads to the reduced growth rates of juvenile organisms. Because small adult size typically implies low fitness (Honˇ ek, 1993; Shine, 1994; Head, 1995; Blanckenhorn, 2000; Isaac, 2005), slowly-growing juve- niles often prolong their growth periods to compensate for the reduced growth rate. This results in a negative relationship between pupal weight and larval development time. Such neg- ative, nonlinear relationships between age and size at maturity are predicted theoretically (Stearns, 1992; Day & Rowe, 2002; Roff, 2002) and often termed as ‘L-shaped reaction norms’. Prolonging the typically vulnerable juvenile stage is, naturally, Correspondence: Toomas Esperk, Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 46, EE-51014 Tartu, Estonia. Tel.: +372-53-302-480; e-mail: tome@ut.ee not free of associated costs (Lima & Dill, 1990; Kozlowski, 1992; Blanckenhorn, 2000; Gotthard, 2001, 2008). As an alter- native, a growing juvenile may compensate by increasing its growth rate after the period of nutritional deficit has passed. Indeed, such a compensatory, or catch-up growth, is observed in various organisms (Metcalfe & Monaghan, 2001; Ali et al., 2003; Mangel & Munch, 2005). However, the accelerated growth appears to have various costs (Gotthard et al., 1994; Arendt, 1997; Nylin & Gotthard, 1998; Gotthard, 2000; Fischer et al., 2004; Stoks et al., 2005, 2006; De Block & Stoks, 2008a, b). The optima for the degree of compensation, and the set of proximate mechanisms used, should necessarily depend on fit- ness functions of body size, development time and growth rate. In particular, a high degree of compensation is expected if final size is closely associated with fitness, and/or compensation is relatively cost-free. Indeed, a broad range of compensatory  2010 The Authors 222 Journal compilation  2010 The Royal Entomological Society